Prior art optical couplers used in photonic integrated circuits (PIC) utilize both a waveguide, e.g., made of silicon, and a polymer waveguide positioned on top of the waveguide to establish evanescent coupling. An optical fiber is then optically coupled to the polymer waveguide for providing optical signals to or from the photonic integrated circuit. The use of the polymer waveguide on the PIC can lead to absorption loss within the polymer waveguide that deteriorates the optical signal. For example, optical attenuation in the polymer waveguide may lead to a loss of about 0.5 dB and coupling between the polymer waveguide and the optical fiber may result in an additional 1 dB of loss.
An alternative to using polymer core and polymer clad waveguides for evanescent optical couplers involves using polymer clad optical fibers having an inner glass cladding that surrounds a glass core and an outer polymer cladding that surrounds the inner glass cladding. The outer polymer cladding keeps the optical mode well bound for low loss propagation but is easily stripped off in a desired location so that the fiber core can be brought into close proximity to the core of another waveguide (e.g., silicon or even another optical fiber) to establish evanescent coupling between the two waveguides.
Efficient evanescent coupling between a polymer clad fiber and a waveguide requires that the separation between the two waveguides needs to be controlled to challenging tolerances, e.g., to micron or even sub-micron levels. Even more challenging is assembling multiple polymer clad fibers to multiple waveguides with this same separation tolerance in a high throughput manufacturing environment. Another challenge is ensuring that the fiber core, which usually has a flat section, is it not tilted or otherwise angularly offset from the waveguide to which the fiber is being evanescently coupled.